The invention relates to an optical system for slitwise exposure which is adapted for use in a copying machine.
A conventional optical system for slitwise exposure which is used in an electrostatic copying machine may comprise an arrangement as shown in FIG. 1 in which an original is placed on an original receptacle which comprises a transparent glass pane 2. A first mirror M1 is moved in the direction of an arrow a, and a second mirror M2 is moved in the same direction with one-half the speed of the first mirror M1, with an exposure radiation from the first mirror M1 being introduced through an inmirror lens Ml onto a photosensitive sheet or a photosensitiive drum. Alternatively, FIG. 2 shows an arrangement in which the first mirror M1 is moved in the direction of the arrow a parallel to the original receptacle 2 with a speed V as before, and the second mirror M2 is moved in a direction indicated by an arrow b, which is parallel to the path of the flux exiting therefrom, with a speed of 1/2V sec .theta. (.theta. being the angle of reflection of the emitting flux), with the flux from the second mirror M2 being introduced through an in-mirror lens onto a photosensitive drum.
A copying machine which employs an optical system of the movable mirror type for slitwise exposure has a physical size which is determined principally by the extent of the movement of the optical system. Where the original receptacle is stationary, the size of the original receptacle will determine the minimum achievable size for the top of the copying machine, which can be attained by arranging the slitwise exposure system such that its extent of movement does not extend beyond the space occupied by the original receptacle 2. To this end, an arrangement must be made such that the second mirror M2 moves into coincidence with the position of the first mirror M1 or short thereof when the first mirror M1 has moved to the end of the original receptacle 2. With the conventional optical system mentioned above, the scanning flux from the first mirror M1 cannot be passed to the in-mirror lens Ml unless the second mirror M2 maintains a position which is forwardly of the first mirror M1 during the scanning of the original, so that, in the arrangement shown in FIG. 1 or FIG. 2, when the first mirror M1 moves from a starting end of the original receptacle 2 (shown in solid line) to its terminal end (shown in broken lines) through a distance l in order to complete a scanning of the original 1, the second mirror M2 will assume a position (shown in broken lines) which is displaced by a distance .DELTA.l from the terminal end of the original receptacle 2. As a consequence, the extent of movement of the optical system exceeds the space occupied by the original receptacle 2, so that a copying machine incorporating such an optical system will have to be made oblong in the transverse dimension, resulting in a relatively bulky structure.